Conventionally, welding apparatuses for performing butt welding of a tube by the use of a welding torch rather than laser welding have been known (for example, see Patent Literature 1). In this welding apparatus, multi-layer welding is performed on a groove portion formed by butting the tubes, while supplying a wire and a shield gas to the welding torch. That is, the welding apparatus performs the welding on the groove portion by a plurality of welding passes. At this time, in order to suppress the slope deformation (a welding falling deformation) of the other tube with respect to one tube due to the welding, the welding apparatus measures an amount of deformation after welding of the first layer, and sets a starting point of welding or heating of the next layer to an opposite position of the maximum displacement point on the basis of the measured amount of deformation.